The present invention relates to techniques for etching through a metallization layer on a substrate. More particularly, the present invention relates to techniques for etching through the metallization layer while reducing corrosion due to byproducts produced during prior art metallization etch processes.
In the fabrication of semiconductor integrated circuits, metal lines are often employed as conductive paths between the devices on the integrated circuit. To form the metal lines and features, a metal layer is typically blanket deposited over the surface of the wafer. Using an appropriate photoresist mask, portions of the metal layer are then etched away, leaving behind metal lines and features.
As the density of integrated circuits increases and the feature sizes decrease, a variety of techniques has been developed to properly etch the ever shrinking features of the integrated circuit. One of these techniques involves plasma-enhanced etching. To facilitate discussion, FIG. 1 depicts a metal layer 102 disposed on a substrate 104. Substrate 104 may represent the wafer itself or, more commonly, a layer upon which metal layer 102 is disposed such as an oxide layer. In the example of FIG. 1, metal layer 102 includes a barrier layer 106, typically formed of a material such as titanium. In some cases, barrier layer 106 may represent a composite layer, which includes a titanium nitride (TiN) overlaying a titanium layer. Metal layer 108 is typically formed of aluminum or one of its alloys such as aluminum/copper or aluminum/copper/silicon. Between metal layer 108 and photoresist mask 110, there is disposed a barrier/ARC (anti-reflective coating) layer 112. Barrier/ARC layer 112 may include, for example, an overlaying anti-reflective coating, which may be organic or inorganic in character. One skilled in the art would recognize that the anti-reflective coating layer is provided primarily for the lithography purposes. A barrier layer of titanium and/or titanium nitride may be disposed under the aforementioned anti-reflective coating layer. Although metal layer 102 is shown including barrier/ARC layer 112, metal layer 108, and barrier layer 106, one skilled in the art would readily recognize that both barrier/ARC layer 112 and barrier layer 106 are optional and one or both may be omitted in some ICs.
Photoresist mask 110 represents a portion of the photoresist mask that has been formed using an appropriate photoresist process. During the etching of metal layer 102, photoresist mask 110 protects the portions of the metal layer 102 disposed below the photoresist features, thereby forming features out of the underlying metal layer. By way of example, the etching of metal layer 102 produces a line disposed perpendicular to the page on which FIG. 1 is depicted.
In FIG. 2, etching is completed and portions of metal layer 102 that are not protected by the photoresist features are removed. Typically, the etching of an aluminum-containing metal layer is accomplished in a plasma reactor using, for example, etching source gases such as Cl.sub.2 /BCl.sub.3, CL.sub.2 /HCl, CL.sub.2 /N.sub.2, and the like. The etching may be performed in accordance with a plasma-enhanced process known as reactive ion etching (RIE), for example. In FIG. 2, there are shown polymer side walls 202A and 202B coating the vertical surfaces of metal feature 204. Typically, the polymer side walls contain organic materials such as sputtered photoresist from photoresist mask 110, resputtered material from the metallization layers (such as aluminum, titanium, and the like), material sputtered from the underlying layer (such as from substrate 104) and a nontrivial amount of chlorine and/or chlorine-containing compounds from the etching source gas. As will be discussed later herein, the polymer side walls need to be removed as part of the post metallization processing steps.
In FIG. 3, the photoresist mask is removed. In a typical plasma-enhanced process, photoresist mask removal may be achieved by stripping away the photoresist material in a downstream ash plasma reactor using, for example, O.sub.2 /H.sub.2 O vapor or O.sub.2 as the ashing material. As is typically the case, the photoresist strip process removes very little, if any, of the side wall polymers. Consequently, the polymer side walls remain after the photoresist strip process and must be removed in a subsequent process step. Complete removal of the polymer side walls is highly desirable because if side wall polymer remains attached to the metal line, the chlorine therein may react with moisture in the ambient environment, forming corrosive acids that attack the metal lines. As can be appreciated by those skilled in the art, erosion of the metal lines changes the electrical characteristics of metal lines, e.g., increasing their resistivity. In some cases, the corrosion may be severe enough to sever the conductive path, forming an open circuit where none is intended.
As mentioned, a separate process is typically required in the prior art to remove the polymer side walls that remain after the photoresist strip process. In the prior art, polymer side wall removal is typically accomplished by a wet etch process since it has been found that plasma-enhanced etching is relatively inefficient in removing the deposited polymer. The wet etch may be preceded by a passivating plasma process and/or a deionized water rinse process. The wet etch process typically employs a suitable wet etchant such as chromic phosphoric, diluted sulfuric peroxide, organic solvents EKC265 from EKC Technology, Inc. of Hayward, Calif. or ACT935 from Ashland Chemical Company of Columbus, Ohio. FIG. 4 shows the metallization feature of FIG. 3 after the wet etch has been performed to remove the polymer side walls.
Although the wet etch process accomplishes its purpose of removing the polymer side wall, there are disadvantages. For example, the wet etch process typically employs corrosive chemicals that typically do not have a high selectivity to aluminum. If the wet etch process is not carefully controlled, the wet etchant can attack the aluminum lines especially at the interface with the upper and/or lower barrier layers or at the interface between an aluminum line and an underlying tungsten stud. The stud-related corrosion is particularly severe if there is any stud that is misaligned with the metal line (due to, e.g., manufacturing tolerance) and is therefore not completely covered by a metal line. On the other hand, the wet etch must be allowed to proceed sufficiently such that substantially all side wall polymers are removed to prevent the aforementioned corrosion problem. Because of the relatively narrow process window, the use of the wet etch process to remove the polymer side walls has posed many challenges to process engineers.